A conventional power stapler is partially disclosed in FIG. 1 and comprises a barrel 15 with a handle connected thereto which has a trigger means 17 connected thereto. A cylinder 16 is fixedly received in the barrel 15 and has a piston 18 movably received therein so as to push an actuating plate 19 connected thereto when the high pressure is introduced into the cylinder 16 via the space defined between the cylinder 16 and the inlet valve. The inlet valve includes a movable cap 12 which is located between the cylinder 16 and the head member 11 which is fixedly attached to the inside of the barrel 15. The head member 11 has a central hole 111 defined in the center thereof so that the neck portion 121 of the cap 12 extends through the central hole 111 with a seal 112 received between the neck portion 121 and the head member 11. The cap 12 further has a flange 122 extending radially inward from the inside of the neck portion 121 so that a spring 13 is biased between the flange 122 and the rear end board 14 of the barrel 15. The cap 12 has two grooves defined in the outside thereof so as to receive a seal 125 therein which is to be moved along the inner surface of a flange 113 when the cap 12 is moved. Yet another seal 126 is mounted to the cap 12 so as to seal the space between the end of the cylinder 16 and the cap 12. When operating the trigger means 17, the high pressure in the space 120 to push the cap 12 toward the cylinder 16 is released via the pipe 170 which is controlled by the trigger means 17 so that the high pressure in the space 150 around the cylinder 16 pushes the cap 12 toward the rear end board 14 and enters into the cylinder 16 to push the piston 18 to send a staple (not shown) by the actuating plate 19. After the piston 18 is pushed, the spring 13 pushes the cap 12 back to its original position again. During the operation, the seal 112, 125 and 126 perform an import role to ensure there has no leak between the head member 11, the cap 12 and the cylinder 16 so as to precisely push the piston 18. However, the seals 112, 125 and 126 then to be worn out because they are moved during each movement of the cap 12. Accordingly, the seals 112, 125 and 126 have to be replaced frequently and this is disadvantageous for the users.
The present invention intends to provide an improved inlet valve which employs a sealing member connected between the movable cap and the fixed head member. The sealing member has a flexible portion which can be extended when the cap is moved so as to seal the space between the head member and the cap without moving. By this structure, the shortcomings of the conventional power stapler can be mitigated.